Thermal flow meter

ABSTRACT

Provided is a thermal flow meter including an inlet-side body into which an. inlet of a measurement tube is inserted, an outlet-side body into which an outlet of the measurement tube is inserted, an inlet-side nut having internal threads fastened to external threads formed on the inlet-side body, an outlet-side nut having internal threads fastened to external threads formed on the outlet-side body, an inlet-side ferrule of resin being deformed to form a seal area as the inlet-side nut  15   a  is fastened to the inlet-side body, and an outlet-side ferrule of resin being deformed to form a seal area as the outlet-side nut is fastened to the outlet-side body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on Japanese Patent Application No.2015-033221, the contents of which are incorporated herein by referencein its entirety.

TECHNICAL FIELD

The present disclosure relates to a thermal flow meter.

BACKGROUND ART

Thermal flow meters are known in the art chat control the temperature ofliquid flowing through, the flow passage to measure a flow rate based ona difference between temperatures of the liquid upstream and downstreamof the temperature control part (refer to Japanese Unexamined PatentApplication, Publication No. 2006-10322, for example).

Japanese Unexamined Patent Application, Publication No. 2006-10322discloses a thermal flow meter with a flow passage a wetted portion ofwhich is entirely formed of glass that is obtained by forming arectangular groove on a glass substrate and bonding another glasssubstrate with a heat transfer means and a temperature detecting meansto the groove side of the earlier glass substrate.

CITATION LIST Patent Literature

{PTL 1}

Japanese Unexamined Patent Application, Publication No. 2006-10322

SUMMARY Technical Problem

The thermal flow meter desirably includes a tubular measurement tubewith a straight tube portion extending linearly when measuring a verysmall amount of flow rate (e.g., 0.1 cc/min to 30 cc/min). The inclusionof the tubular measurement tube with the straight tube portion enablesrectifying flow of liquid to stably send the liquid to the temperaturedetecting means. Moreover, the liquid is replaced with another liquidwell without any bubbles remaining on the flow passage to have enhancedliquid replacement property. Furthermore, the measurement tube, whichhas a relatively simple shape, can be manufactured at a low cost as wellas promoting ease of assembly.

When including the tubular measurement tube with the straight tubeportion, however, it is required to securely fix both end portions ofthe measurement tube with a small diameter and to suppress leakage ofthe liquid at the both ends of the measurement tube.

The present disclosure has been made in view of the circumstances, andaims to provide a thermal flow meter in which the both end portions ofthe measurement tube are securely fixed and which can suppress theleakage of the liquid at the both ends of the measurement tube.

Solution to Problem

In order to solve the foregoing problem, the following solutions havebeen adopted in the present disclosure.

A thermal flow meter according to an aspect of the present disclosureincludes a measurement tube including an inlet through which liquidenters and an outlet through which the liquid flowing from the inletexits, and having an internal flow passage extending along an axis, atemperature detecting substrate having a heating resistance element anda temperature detecting resistance element formed on a detection surfacethereof along the axis, the detection surface being bonded to themeasurement tube along the axis, an inlet-side body which has inside afirst connection flow passage and into which the inlet of themeasurement tube is inserted, an outlet-side body which has inside asecond connection flow passage and into which the outlet of themeasurement tube is inserted, a cylindrical inlet-side nut fitted alongan outer circumferential surface of the measurement tube to be closer tothe outlet than the inlet-side body, the inlet-side nut having on aninner circumferential surface thereof internal threads fastened toexternal threads formed on an outer circumferential surface of theinlet-side body, a cylindrical outlet-side nut fitted along the outercircumferential surface of the measurement tube to be closer to theinlet than the outlet-side body, the outlet-side nut having on an innercircumferential surface thereof internal threads fastened to externalthreads formed on an outer circumferential surface of the outlet-sidebody, an inlet-side ferrule of resin formed in a cylindrical shape andfitted between the outer circumferential surface of the measurement tubeand an inner circumferential surface of an end portion of the inlet-sidebody facing the outlet, the inlet-side ferrule being deformed to form aseal area as the inlet-side nut is fastened to the inlet-side body, andan outlet-side ferrule of resin formed in a cylindrical shape and fittedbetween the outer circumferential surface of the measurement tube and aninner circumferential surface of an end portion of the outlet-side bodyfacing the inlet, the outlet-side ferrule being deformed to form a sealarea as the outlet-side nut is fastened to the outlet-side body.

According to a thermal flow meter in an aspect of the presentdisclosure, the inlet of the measurement tube with the detection surfaceof the temperature detecting substrate bonded thereto is inserted intothe inlet-side body and connected to the first connection flow passageformed inside the inlet-side body. Similarly, the outlet of themeasurement tube is inserted into the outlet-side body and connected tothe second connection flow passage formed inside the outlet-side body.

When the external threads formed on the outer circumferential surface ofthe inlet-side body are fastened to the internal threads formed on theinner circumferential surface of the inlet-side nut, the inlet side endportion of the measurement tube is fixed, and at the same time, thecylindrical inlet-side ferrule that is fitted around the outercircumferential surface of the measurement tube is deformed to form theseal area. Similarly, when the external threads formed on the outercircumferential surface of the outlet-side body are fastened to theinternal threads formed on the inner circumferential surface of theoutlet-side nut, the outlet side end portion of the measurement tube isfixed, and at the same time, the cylindrical outlet-side ferrule that isfitted around the outer circumferential surface of the measurement tubeis deformed to form the seal area.

The formation of the seal area on the inlet side of the measurement tubeprevents the liquid from flowing out through the location of connectionof the internal flow passage of the measurement tube with the firstconnection flow passage of the inlet-side body. Similarly, the formationof the seal area on the outlet side of the measurement tube prevents theliquid from flowing out through the location of connection of theinternal flow passage of the measurement tube with the second connectionflow passage of the outlet-side body.

Thus, according to a thermal flow meter in an aspect of the presentdisclosure, there can be provided the thermal flow meter in which theboth end portions of the measurement tube are securely fixed and whichcan suppress the leakage of the liquid at the both ends of themeasurement tube.

A thermal flow meter according to an aspect of the present disclosuremay be configured such that an end portion of the inlet-side nut facingthe outlet is provided with a first recess into which an end portion ofthe temperature detecting substrate facing the inlet is inserted, theend portion of the temperature detecting substrate facing the inlet isfixed to the inlet-side nut by a filler charged in the first recess, anend portion of the outlet-side nut facing the inlet is provided with asecond recess into which an end portion of the temperature detectingsubstrate facing the outlet is inserted, and the end portion of thetemperature detecting substrate facing the outlet is fixed to theoutlet-side nut by a filler charged in the second recess.

With this configuration, the end portion of the temperature detectingsubstrate facing the inlet is fixed, by the filler, inside the firstrecess of the inlet-side nut, and the end portion of the temperaturedetecting substrate facing the outlet is fixed, by the filler, insidethe second recess of the outlet-side nut. This can securely fix the bothend portions of the measurement tube to the inlet-side nut and theoutlet-side nut.

A thermal flow meter according to an aspect of the present disclosure isconfigured such that the measurement tube has on an outercircumferential surface thereof a flat surface on which the detectionsurface of the temperature detecting substrate is oppositely positioned,and the flat surface and the detection surface of the temperaturedetecting: substrate are bonded together with an adhesive.

This enhances the adhesiveness of the temperature detecting substrate tothe measurement tube while reducing the amount of the adhesive requiredfor the adhesion.

In a thermal flow meter in accordance with an aspect of the presentdisclosure, a first distance from the detection surface of thetemperature detecting substrate to an inner circumferential surface ofthe internal flow passage may be shorter than a second distance from atop portion of the measurement tube to the inner circumferential surfaceof the internal flow passage.

This can enhance the property of the heating resistance element to heatthe liquid inside the internal flow passage and the property of thetemperature detecting resistance element to detect the temperature ofthe liquid.

In a thermal flow meter in accordance with an aspect of the presentdisclosure, the temperature detecting substrate and the measurement tubemay be made of glass.

Using the temperature detecting substrate and the measurement tube whichhave excellent heat resistance and less probability of deformation byheat, the flow rate measurement accuracy can be maintained regardless ofchanges in temperature. In addition, because the materials with the samethermal conductivity properties are bonded together, the adhesiveness ofthe temperature detecting substrate to the measurement tube can bemaintained regardless of changes in temperature.

Advantageous Effects

The present disclosure can provide a thermal flow meter in which bothend portions of a measurement tube are securely fixed and which cansuppress leakage of the liquid at both ends of the measurement tube.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a vertical cross-sectional view of a thermal flow meteraccording to a first embodiment;

FIG. 2 is an exploded assembly view of the thermal flow meter shown inFIG. 1;

FIG. 3 is a vertical cross-sectional view illustrating a sensor unit,shown in FIG. 2;

FIG. 4 is an exploded assembly view of the sensor unit shown in FIG. 3;

FIG. 5A illustrates a plan view of a measurement tube, a sensorsubstrate, and a reinforcing plate shown in FIG. 3;

FIG. 5B illustrates a vertical cross-sectional view of a measurementtube, a sensor substrate, and a reinforcing plate shown in FIG. 3;

FIG. 5C illustrates a bottom view of a measurement tube, a sensorsubstrate, and a reinforcing plate shown in FIG. 3;

FIG. 6 is a cross-sectional view of the sensor unit shown in FIG. 3taken along the line A-A;

FIG. 7 is a cross-sectional view of the measurement tube, the sensorsubstrate, and the reinforcing plate shown in FIG. 5B taken along theline B-B;

FIG. 8 is a plan view of a detection surface of the sensor substrateshown in FIG. 5B;

FIG. 9 is a vertical cross-sectional view of the sensor unit of thethermal flow meter according to a second embodiment; and

FIG. 10 is an elevational view of a guide shown in FIG. 9.

DESCRIPTION OF EMBODIMENTS First Embodiment

Hereinafter, a thermal flow meter 100 according to a first embodiment ofthe present disclosure will be described with reference to the drawings.

The thermal flow meter 100 of the embodiment heats liquid flowingthrough an internal flow passage and detects a temperature of the heatedliquid to thereby measure a flow rate thereof. The thermal flow meter100 of the embodiment is suitable for measuring a very small amount offlow rate in the range of from 0.1 cc/min to 30 cc/min, for example.

As shown in FIGS. 1 and 2, the thermal flow meter 100 of the embodimentincludes a sensor unit 10, a control substrate 20, a relay substrate 30,an upper case 40, and a bottom case 50.

As shown in FIG. 3, the sensor unit 10 lets incoming liquid from aninlet 10 a connected to external piping (not shown) flow out through anoutlet 10 b connected to external piping (not shown), and at the sametime, measures a flow rate of the liquid flowing through an internalflow passage 10 c. The sensor unit 10 does not directly calculate theflow rate of the liquid, but detects the temperature of the liquidheated by a heating resistance wire 12 a (a heating resistance element)that will be described later with temperature detecting resistance wires12 b and 12 c (temperature detecting resistance elements), and transmitsa temperature detection signal indicating the detected, temperature tothe control, substrate 20 through a signal wire (not shown).

The sensor unit 10 will, be described later in detail.

The control substrate 20 transmits a voltage signal to the heatingresistance wire 12 a of the sensor unit 10 to heat it, and alsocalculates the flow rate of the liquid based on the temperaturetransmitted from the temperature detecting resistance wires 12 b and 12c.

The relay substrate 30 that relays various signals transmitted andreceived between the control substrate 20 and an external device (notshown). A cable 200 for transmitting and receiving the various signalsto and from the external device (not shown) is connected to the relaysubstrate 30.

The upper case 40 serves as a housing for an upper portion of thethermal flow meter 100, and accommodates the control substrate 20inside.

The bottom case 50 serves as a housing for a lower portion of thethermal flow meter 100, and accommodate the sensor unit 10 inside. Withthe sensor unit 10 inserted in the bottom case 50, a stopper 60 isinserted between the bottom case 50 and the sensor unit 10 from theinlet 10 a side of the sensor unit 10.

Similarly, with the sensor unit 10 inserted in the bottom case 50, astopper 70 is inserted between the bottom case 50 and the sensor unit 10from the outlet 10 b side of the sensor unit 10. The sensor unit 10becomes fixed to the bottom case 50 by means of the stoppers 60 and 70.

The bottom case 50 has fastening holes 50 a on its bottom surface and isfixed to an installation surface (not shown) by fastening bolts (notshown) that are inserted from below the installation surface.

Next, the sensor unit 10 will be described in detail with reference toFIGS. 3 to 8.

As shown in FIGS. 3 and 4, the sensor unit 10 has a measurement tube 11,a sensor substrate 12 (temperature detecting substrate), a reinforcingplate 13, a guide 14, a nut 15, an inlet-side body 16, an outlet-sidebody 17, an inlet-side ferrule 18, and an outlet-side ferrule 19.

The measurement tube 11 is a tube that includes an inlet 11 a throughwhich liquid enters and an outlet 11 b through which the liquid from theinlet 11 a exits. As shown in FIG. 6 (a cross-sectional view taken alongthe line A-A in FIG. 3), the measurement tube 11 has formed therein theinternal flow passage 10 c that extends along an axis X and has acircular cross section. The measurement tube 11 is formed of a resinmaterial with high corrosion resistance to alkaline liquids (e.g., PTFE:polytetrafluoroethylene).

As shown in FIG. 8, the sensor substrate 12 is a glass substrate (e.g.,made of quarts glass, which has a high silicon dioxide content) with theheating resistance wire 12 a (the heating resistance element), thetemperature detecting resistance wire 12 b (the temperature detectingresistance element), and the temperature detecting resistance wire 12 c(the temperature detecting resistance element) formed on a detectionsurface 12 d along the axis X.

The heating resistance wire 12 a, the temperature detecting resistancewire 12 b, and the temperature detecting resistance wire 12 c are eachformed of a metal film, such as of platinum, evaporated onto the glasssubstrate.

The liquid flowing in the measurement tube 11 flows along the axis Xfrom the left to the right in FIG. 8. Accordingly, when the heatingresistance wire 12 a is heated momentarily, the heated liquid flowsalong the axis X to the position of the temperature detecting resistancewire 12 b and then to the temperature detecting resistance wire 12 c.

The control substrate 20, therefore, can calculate the flow speed of theliquid flowing in the measurement tube 11 from the timing at which theheating resistance wire 12 a was momentarily heated and the timings atwhich the temperature detecting resistance wire 12 b and the temperaturedetecting resistance wire 12 c subsequently detect the temperature ofthe heated liquid. Also, the control substrate 20 can calculate the flowrate of the liquid from the obtained flow speed and the cross-sectionalarea of the measurement tube 11.

Although the temperature detecting resistance wires 12 b and 12 c arearranged downstream of the heating resistance wire 12 a in the liquidflowing direction in FIG. 8, they may be arranged otherwise.

For example, the temperature detecting resistance wire 12 b may bearranged upstream of the heating resistance wire 12 a in the liquidflowing direction, and the temperature detecting resistance wire 12 cmay be arranged downstream of the heating resistance wire 12 a in theliquid flowing direction. The temperature distribution, to be created bythe heating resistance wire 12 a depends on the flowing speed of theliquid, and the more the flowing speed increases, the more heat isconveyed downstream to raise the downstream temperature. The controlsubstrate 20 can calculate the flow rate of the liquid from a differencebetween temperatures detected by the temperature detecting resistancewires 12 b and 12 c, and a cross-sectional area of the measurement tube11.

As shown in FIG. 6 (the cross-sectional view taken along the line A-A inFIG. 3), the measurement tube 11 has an approximately circular crosssection in a plane orthogonal to the axis X, where the sensor substrate12 is bonded to the measurement tube 11. An outer circumferentialsurface of the measurement tube 11 has a flat surface 11 c where thedetection surface 12 d of the sensor substrate 12 is arranged as opposedto the measurement tube 11.

On the other hand, as shown in FIG. 7 (a cross-sectional view takenalong the line BB in FIG. 5(B)), the measurement tube 11 has a circularcross section in a plane orthogonal to the axis X, where the sensorsubstrate 12 is not bonded to the measurement tube 11.

As shown in FIGS. 5 and 6; the flat surface 11 c for the sensorsubstrate of the measurement tube 11 is arranged to be opposed to thedetection surface 12 d of the sensor substrate 12. The flat surface 11 cfor the sensor substrate and the detection surface 12 d axe bondedtogether with an adhesive.

Here, examples of the adhesive may include epoxy resin-based adhesives,UV curable resin-based adhesives, thermosetting-resin-based adhesives(thermosetting adhesives), and low melting point glasses.

As shown in FIG. 6, a distance D1 (a first distance) from the detectionsurface 12 d of the sensor substrate 12 to an inner circumferentialsurface 10 d of the internal flow passage 10 c is made shorter than adistance D2 (a second distance) from the top portion 11 d of themeasurement tube 11 to the inner circumferential surface 10 d of theinternal flow passage 10 c. This is for the purpose of improving thermalconductivity from the heating resistance wire 12 a to the liquid whileimproving temperature detecting property of the temperature detectingresistance wire 12 b and the temperature detecting resistance wire 12 c,by reducing the distance B1 from the detection surface 12 d of thesensor substrate 12 to the inner circumferential surface 10 d of theinternal flow passage 10 c.

As shown In FIG. 6, the guide 14 is a metal member (e.g., made ofstainless steel) that has a circular cross section and is provided withan opening portion 14 a at its top.

The guide 14 guides a pair of nuts 15 such that the nuts are coupledtogether. As shown in FIG. 3, the pair of nuts 15 are coupled togethervia the measurement tube 11, and the measurement tube 11 and the nuts 15are fixed together by a filler 15 i that is charged into a recess 15 e(a first recess) and a filler 15 j that is charged into a recess 15 f (asecond recess).

Accordingly, a space between the pair of nuts 15 in the direction of theaxis X is fixed in the sensor unit 10 after manufacture shown in FIG. 3.On the other hand, the measurement tube 11 and the nuts 15 are not fixedtogether when the recesses 15 e and 15 f are not filled with the fillers15 i and 15 j. With the measurement tube 11 and the nuts 15 not fixedtogether, the guide 14 is in contact at one end portion with a stepportion 15 c of the inlet-side nut 15 a and at the other end portionwith a step portion 15 d of the outlet-side nut 15 b to thereby maintaina constant distance between the pair of nuts 15.

Examples of the fillers 15 i and 15 j include those of epoxy resins,acrylic resins, and silicone resins.

The inlet-side body 16 receives the inlet 11 a of the measurement tube11 and has inside a connection flow passage 16 a (a first connectionflow passage) that has a circular cross section, as shown in FIG. 3. Theinlet-side body 16 has external threads 16 b on an outer circumferentialsurface of its end portion facing the outlet 10 b.

The outlet-side body 17 receives the outlet 11 b of the measurement tube11 and has inside a connection flow passage 17 a (a second connectionflow passage) that has a circular cross section, as shown in FIG. 3. Thecutlet-side body 17 has external threads 17 b on an outercircumferential surface of its end portion facing the inlet 10 a.

The inlet-side body 16 and the outlet-side body 17 are formed from aresin material with high corrosion resistance (e.g., PTFE:polytetrafluoroethylene).

The nuts 15 include the inlet-side nut 15 a attached to the inlet-sidebody 16 and the outlet-side nut 15 b attached to the outlet-side body17.

As shown in FIG. 3, the inlet-side nut 15 a is a cylindrical memberfitted along the outer circumferential surface of the measurement tube11 to be closer to the outlet 11 b than the inlet-side body 16. Theinlet-side nut 15 a has internal threads 15 g on an innercircumferential surface of its end portion facing the 10 a. Also, theoutlet-side nut 15 b is a cylindrical member fitted along the outercircumferential surface of the measurement tube 11 to be closer to theinlet 11 a than the outlet-side body 17. The outlet-side nut 15 b hasinternal threads 15 h on an inner circumferential surface of its endportion facing the outlet 10 b.

The inlet-side nut 15 a is attached to the inlet-side body 16 as theinternal threads 15 g of the inlet-side nut 15 a and the externalthreads 16 b of the inlet-side body 16 are fastened together. Similarly,the outlet-side nut 15 b is attached to the outlet-side body 17 as theinternal threads 15 b of the outlet-side nut 15 b and the externalthreads 17 b of the outlet-side body 17 are fastened together.

The inlet-side nut 15 a has the recess 15 e (the first recess) that isrecessed toward the inlet 10 a, at its end portion facing the outlet 10b. As shown in FIG. 3, an inlet 11 a side end portion of the sensorsubstrate 12 and an inlet 11 a side end portion of the reinforcing plate13 are inserted in the recess 15 e. Also, the recess 15 e is filled withthe filler 15 i. The inlet 11 a side end portion of the sensor substrate12 and the inlet 11 a side end portion of the reinforcing plate 13 arefixed to the inlet-side nut 15 a by the filler 15 i.

The outlet-side nut 15 b has the recess 15 f (the second recess) that isrecessed toward the outlet 10 b, at its end portion facing the inlet 10a. As shown in FIG. 3, an outlet 11 b side end portion, of the sensorsubstrate 12 and an outlet 11 b side end portion of the reinforcingplate 13 are inserted in the recess 15 f. Also, the recess 15 f isfilled with the filler 15 j. The outlet 11 b side end portion of thesensor substrate 12 and the outlet 11 b side end portion of thereinforcing plate 13 are

It is to be noted that although, the fillers are only charged, into therecesses 15 e and 15 f of the nuts 15 in the foregoing description, thesensor unit 10 may have other configurations. For example, the fillermay be charged to fill up the entire region inside an innercircumference of the guide 14 in addition to the recesses 15 e and 15 f.In that case, the sensor substrate 12 and the reinforcing plate 13 areentirely fixed with the filler, and at the same time, the inlet-side nut15 a and the outlet-side nut 15 b are fixed together by the filler.

The inlet-side ferrule 18 is a cylindrical resin member (e.g., formed ofFTFE) that is inserted in between the outer circumferential surface ofthe measurement tube 11 and the inner circumferential surface of theoutlet 10 b side end portion of the inlet-side body 16.

As shown in FIG. 4, an inlet 10 a side end portion of the inlet-sideferrule 18 has a tip portion 18 a at which the distance between an innercircumferential surface and an outer circumferential surface of the tipportion 18 a gradually decreases toward the inlet 10 a. The tip portion18 a is inserted into a groove portion 16 e formed inside the inlet-sidebody 16 as it is inserted into the inlet-side body 16.

The outlet-side ferrule 19 is a cylindrical resin member (e.g., formedof PTFB) that is inserted in between the outer circumferential surfaceof the measurement tube 11 and an inner circumferential surface of theinlet 10 a side end portion of the outlet-side body 17.

As shown in FIG. 4f an outlet 10 b side end portion of the outlet-sideferrule 19 has a tip portion 19 a at which the distance between an innercircumferential surface and an outer circumferential surface graduallydecreases toward the outlet 10 b. The tip portion 19 a is inserted intoa groove portion 17 c formed inside the outlet-side body 17 as it isinserted into the outlet-side body 17.

As shown in FIG. 4, the groove portion 16 c of the inlet-side body 16and the groove portion 17 c of the outlet-side body 17 are each formedto decrease in the groove width with the distance from the entrance ofthe groove to the bottom portion, The groove portion 16 c and the tipportion 18 a have the same length in the direction of the axis X, whilethe groove portion 16 c is sharper them the tip portion 18 a. Thus, inorder to accommodate the tip portion 18 a in the groove portion 16 cwithout leaving any space, the tip portion 18 a needs to be deformed toconform to the shape of the groove portion 16 c.

Similarly, the groove portion 17 c and the tip portion 19 a have thesame length, in the direction of the axis X, while the groove portion 17c is sharper than the tip portion 19 a. Thus, in order to accommodatethe tip portion 19 a in the groove portion 17 c without leaving anyspace, the tip portion 19 a needs to be deformed to conform to the shapeof the groove portion 17 c.

The sensor unit 10 in the thermal flow meter 100 of the embodiment, isassembled by fastening the internal threads 15 g of the inlet-side nut15 a to the external threads 16 b of the inlet-side body 16 with theinlet 11 a of the measurement tube 11 and the inlet-side ferrule 18inserted in the outlet 10 b side end portion of the inlet-side body 16,and fastening the internal threads 15 h of the outlet-side nut 15 b tothe external threads 17 b of the outlet-side body 17 with the outlet 11b of the measurement tube 11 and the outlet-side ferrule 19 inserted inthe inlet 10 a side end portion of the outlet-side body 17.

The tip portion 18 a or the inlet-side ferrule 18 is forced into thegroove portion 16 c of the inlet-side body 16 as the internal threads 15g of the inlet-side nut 15 a become fastened to the external threads 16b of the inlet-side body 16. Because the groove portion 16 c is sharperthan the tip portion 18 a, the tip portion 18 a is gradually deformed asit is forced into the groove portion 16 c, and finally, deformed to beaccommodated, in the groove portion 16 c without leaving any space.

The deformation of the tip portion 18 a forms a seal area between theouter circumferential surface of the measurement tube 11 and the innercircumferential surface of the inlet-side body 16, which reliably shutsoff liquid that flows out through a location of connection of theconnection flow passage 16 a with the internal flow passage 10 c so thatthe liquid never leaks to the outside. Moreover, the tip portion 18 a ofthe inlet-side ferrule 18 is positioned in the vicinity of the locationof connection of the connection flow passage 16 a with the internal flowpassage 10 c, thereby reducing an amount of liquid that flows outthrough the connection location to be remained (dead volume).

The inlet-side body 16 and the measurement tube 11 are connected suchthat they are arranged with their central axes coincident on the axis X.The connection structure enables the inlet side body 16 and themeasurement tube 11 to be joined together without any steps betweentheir inner walls, so that the flow of the liquid flowing inside willnever be turbulent. Accordingly, the sensor substrate 12 can stablymeasure the flow rate of the liquid.

The fastening of the internal threads 15 g of the inlet-side nut 15 aand the external threads 16 b of the inlet-side body 16 is completed asan inlet 10 a side end of the inlet-side nut 15 a comes into contactwith a projecting portion 16 d of the inlet-side body 16. The amount ofdeformation of the tip portion 18 a forced into the groove portion 16 ccan be kept appropriate by arranging the projecting portion 16 d at anappropriate position.

The tip portion 19 a of the outlet-side ferrule 19 is forced into thegroove portion 17 c of the outlet-side body 17 as the internal threads15 h of the outlet-side nut 15 b become fastened to the external threads17 b of the outlet-side body 17. Because the groove portion 17 c issharper than the tip portion 19 a, the tip portion 19 a is graduallydeformed as it is forced into the groove portion 17 c, and finally,deformed to be accommodated in the groove portion 17 c without leavingany space.

The deformation of the tip portion 19 a forms a seal area between theouter circumferential surface of the measurement tube 11 and the innercircumferential surface of the outlet-side body 17, which reliably shutsoff liquid that flows out through a location of connection of theconnection flow passage 17 a (the second connection flow passage) withthe internal flow passage 10 c so that the liquid never leaks to theoutside. Moreover, the tip portion 19 a of the outlet-side ferrule 19 ispositioned in the vicinity of the location of connection of theconnection flow passage 17 a with the internal flow passage 10 c,thereby reducing an amount of liquid that flows out through theconnection location to be remained (dead volume).

The fastening of the internal threads 15 h of the outlet-side nut 15 band the external threads 17 b of the outlet-side body 17 is completed asan outlet 10 b side end of the outlet-side nut 15 b comes into contactwith a projecting portion 17 d of the outlet-side body 17. The amount ofdeformation of the tip portion 19 a forced into the groove portion 17 ccan be kept appropriate by arranging the projecting portion 17 d at anappropriate position.

As shown in FIG. 5A, a distance L3 from the inlet 11 a of themeasurement tube 11 to the middle portion of the sensor substrate 12 inthe direction of the axis X is longer than a distance L4 from the outlet11 b of the measurement tube 11 to the middle portion of the sensorsubstrate 12 in the direction of the axis X.

This is for the purpose of increasing the distance L3 from the locationof connection of the connection flow passage 16 a with the inlet 11 a ofthe measurement tube 11 to the middle portion of the sensor substrate 12in the direction of the axis X. Even if turbulence is generated in theliquid flow at the connection flow passage 16 a and the inlet 11 a ofthe measurement tube 11, the liquid flow can be stabilized before itreaches the middle portion of the sensor substrate 12 by increasing thedistance L3.

As shown in FIGS. 3 and 4, the length of the inlet-side ferrule 18 alongthe axis X is made longer than the length of the outlet-side ferrule 19along the axis X as the distance L3 is made longer than the distance L4.

This is for the purpose of positioning the tip portion 18 a of theinlet-side ferrule 18 in the vicinity of the location of connection orthe connection flow passage 16 a with the inlet 11 a while positioningthe tip portion 19 a of the outlet-side ferrule 19 in the vicinity ofthe location of connection of the connection flow passage 17 a with theoutlet 11 b.

Next, a method of manufacturing the thermal, flow meter 100 of theembodiment will be described.

To begin with, a description will be given on a method for manufacturingthe sensor unit 10 of the thermal flow meter 100.

First, the adhesive is applied to either one of the detection surface 12d of the sensor substrate 12 and the flat surface 11 c for the sensorsubstrate of the measurement tube 11, and then, the sensor substrate 12and the measurement tube 11 are arranged such that the detection surface12 d and the fiat surface 11 c for the sensor substrate are in contactwith each other via the adhesive, as shown in FIG. 5B.

Second, the measurement tube 11 with the sensor substrate 12 and thereinforcing plate 13 bonded thereto is mounted to a jig (not shown) tosolidify the adhesive. The solidification of the adhesive renders thesensor substrate 12 and the reinforcing plate 13 secured to themeasurement tube 11.

Here, when using a thermosetting-resin-based adhesive as the adhesive,the thermosetting-resin-based adhesive is solidified by mounting themeasurement tube 11 with the sensor substrate 12 and the reinforcingplate 13 bonded thereto to the jig (not shown) and heating them.

Third, the inlet 11 a of the measurement tube 11 is inserted into theinlet-side nut 15 a, the inlet-side ferrule 18, and the inlet-side body16 in this order. Also, the outlet 11 b of the measurement tube 11 isinserted into the outlet-side nut 15 b, the outlet-side ferrule 19, andthe outlet-side body 17 in this order. Here, the inlet-side nut 15 a isattached to the inlet 10 a side end portion of the guide 14 and theoutlet-side nut 15 b is attached to the outlet 10 b side end portion ofthe guide 14, so that the inlet-side nut 15 a and the outlet-side nut 15b are retained, as appropriate.

Fourth, the internal threads 15 g of the inlet-side nut 15 a is fittedaround the external threads 16 b of the inlet-side body 16, and fastenedto the external threads 16 b until the inlet 10 a side end portion ofthe inlet-side nut 15 a comes into contact with the projecting portion16 d. Also, the internal threads 15 h of the outlet-side nut 15 b isfitted around the external threads 17 b of the outlet-side body 17, andfastened to the external threads 17 b until the outlet 10 b side endportion of the outlet-side nut 15 b comes into contact with theprojecting portion 17 d. Here, the tip portion 18 a of the inlet-sideferrule 18 and the tip portion 19 a of the outlet-side ferrule 19 areeach deformed to form the seal areas.

Fifth, the recess 15 e at the outlet 10 b side end portion of theinlet-side nut 15 a is filled with the heated and softened filler 15 i,which is then cooled down until the filler 15 i solidifies. Similarly,the recess 15 f at the inlet 10 a side end portion of the outlet-sidenut 15 b is filled with the heated and softened filler 15 j, which, isthen cooled down until the filler 15 j solidifies.

The sensor unit 10 shown in FIG. 3 is manufactured in this way

Sixth, the sensor unit 10 is inserted into the bottom case 30, and thestoppers 60 and 70 are inserted in between the bottom case 50 and thesensor unit 10. Thus, the sensor unit 10 is fixed to the bottom case 50.

Seventh, the control substrate 20 and the relay substrate 30 areattached to the upper case 40.

Finally, the upper case 40 is attached to the bottom case 50.

The thermal flow meter 100 of the embodiment is manufactured by theforegoing processes.

A description will be given to the operation and effect of the thermalflow meter 100 of the embodiment described above.

According to the thermal flow meter 100 of the embodiment, the inlet 11a of the measurement tube 11 with the detection surface 12 d of thesensor substrate 12 bonded thereto is inserted into the inlet-side body16 and connected to the connection flow passage 16 a formed inside theinlet-side body 16. Similarly, the outlet 11 b of the measurement tube11 is inserted into the outlet-side body 17 and connected to theconnection flow passage 17 a formed inside the outlet-side body 17.

When the external threads 16 b formed on the outer circumferentialsurface of the inlet-side body 16 are fastened to the internal threads15 g formed on the inner circumferential surface of the inlet-side nut15 a, the inlet 11 a side end portion of the measurement tube 11 isfixed, and at the same time, the tip portion 18 a of the cylindricalinlet-side ferrule 18 that is fitted around the outer circumferentialsurface of the measurement tube 11 is deformed to form the seal area.Similarly, when the external threads 17 b formed on the outercircumferential surface of the outlet-side body 17 are fastened to theinternal threads 15 h formed on the inner circumferential surface of theoutlet-side nut 15 b, the outlet 11 b side end portion of themeasurement tube 11 is fixed, and at the same time, the tip portion 19 aof the cylindrical outlet-side ferrule 19 that is fitted around theouter circumferential surface of the measurement tube 11 is deformed toform the seal area.

The formation of the seal area on the inlet 11 a side of the measurementtube 11 prevents the liquid from flowing out through the location ofconnection of the internal flow passage 10 c of the measurement tube 11with the connection flow passage 16 a of the inlet-side body 16.Similarly, the formation of the seal area on the outlet 11 b side of themeasurement tube 11 prevents the liquid from flowing out through thelocation of connection of the internal flow passage 10 c of themeasurement tube 11 with the connection flow passage 17 a of theoutlet-side body 17.

Thus, according to the thermal flow meter 100 of the embodiment, therecan be provided the thermal flow meter 100 in which the both endportions of the measurement tube 11 are securely fixed and which cansuppress the leakage of the liquid at the both ends of the measurementtube 11.

According to the thermal flow meter 100 of the embodiment, the endportion of the sensor substrate 12 facing the inlet 11 a is fixed, bythe filler 15 i, inside the recess 15 e of the inlet-side nut 15 a, andthe end portion of the sensor substrate 12 facing the outlet 11 b isfixed, by the filler 15 j, inside the recess 15 f of the outlet-side nut15 b. This can securely fix the both and portions of the measurementtube 11 to the inlet-side nut 15 a and the outlet-side nut 15 b.

According to the thermal flow meter 100 of the embodiment, themeasurement tube 11 has on its outer circumferential surface the flatsurface 11 c on which the detection surface 12 d of the sensor substrate12 is oppositely positioned. The flat surface 11 c and the detectionsurface 12 d of the sensor substrate 12 are bonded together with theadhesive.

This can enhance the adhesiveness of the sensor substrate 12 to themeasurement tube 11 while reducing the amount of the adhesive requiredfor the adhesion.

According to the thermal flow meter 100 of the embodiment, the distanceD1 from the detection surface 12 d of the sensor substrate 12 to theinner circumferential surface 10 d of the internal flow passage 10 c isshorter than the distance D2 from the top portion 11 d of themeasurement tube 11 to the inner circumferential surface 10 d of theinternal flow passage 10 c.

This can enhance the property of the heating resistance wire 12 a toheat the liquid inside the internal flow passage 10 c and the propertiesof the temperature detecting resistance wires 12 b and 12 c to detectthe temperature of the liquid.

According to the thermal flow meter 100 of the embodiment, the sensorsubstrate 12 and the measurement tube 11 are made of glass. Using thesensor substrate 12 and the measurement tube 11 which have excellentheat resistance and less probability of deformation by heat, the flowrate measurement accuracy can be maintained regardless of changes intemperature. In addition, because the materials with the same thermalconductivity properties are bonded together, the adhesiveness of thesensor substrate 12 to the measurement tube 11 can be maintainedregardless of changes in temperature.

Second Embodiment

Next, a thermal flow meter according to a second embodiment of thepresent disclosure will be described using FIGS. 9 and 10.

The second embodiment is a modification of the first embodiment, and issimilar to the first embodiment unless otherwise described hereinafter.

In the sensor unit 10 of the thermal flow meter according to the firstembodiment, the inlet-side nut 15 a has the recess 15 e that faces theoutlet 10 b, and the outlet-side nut 15 b has the recess 15 f that facesthe inlet 10 a.

On the other hand, in a sensor unit 10′ of the thermal flow meteraccording to the embodiment, an inlet-side nut 15′a has a recess 15′aformed by a guide 14′ that faces the outlet 10 b, and an outlet-side nut15′b has a recess 15′f formed, by the guide 14′ that faces the inlet 10a.

The guide 14′ guides the inlet-side nut 15′a and the outlet-side nut15′b such that the nuts are coupled together. As shown in FIG. 9, theinlet-side nut 15′a and the outlet-side nut 15′b are coupled togethervia the measurement tube 11. The measurement tube 11 and the inlet-sidenut 15′a are fixed together by the filler 15 i that is charged into therecess 15′e (the first recess). Also, the measurement tube 11 and theoutlet-side nut 15′b are fixed together by the filler 15 j that ischarged into the recess 15′f (the second recess).

The guide 14′ is a metal member (e.g., made of stainless steel) that hasa circular cross section and is provided with an opening portion 14′a atits top.

As shown in FIG. 10, the guide 14′ does not open upwardly where it formsthe recess 15′e and the recess 15′f, and has the opening portion 14′aonly at the middle portion along the axis X.

It is to be -noted that the shape of the guide 14 of the firstembodiment is illustrated by a broken line in FIG. 10 for comparison.

According to the embodiment, the inlet-side nut 15′a and the outlet-sidenut 15′b are not specially shaped to have a recess at the end portion,and the recess 15′e and the recess 15′f can be formed using the guide14′.

OTHER EMBODIMENTS

The present invention is not limited to the above embodiment, andmodifications may be made as appropriate without departing from thescope of the present invention.

1. A thermal flow meter comprising: a measurement tube including aninlet through which liquid enters and an outlet through which the liquidflowing from the inlet exits, and having an internal flow passageextending along an axis; a temperature detecting substrate having aheating resistance element and a temperature detecting resistanceelement formed on a detection surface thereof along the axis, thedetection surface being bonded to the measurement tube along the axis;an inlet-side body which has inside a first connection flow passage andinto which the inlet of the measurement tube is inserted; an outlet-sidebody which has inside a second connection flow passage and into whichthe outlet of the measurement tube is inserted; a cylindrical inlet-sidenut fitted along an outer circumferential surface of the measurementtube, to be closer to the outlet than the inlet-side body, theinlet-side nut having on an inner circumferential surface thereofinternal threads fastened to external threads formed on an outercircumferential surface of the inlet-side body; a cylindricaloutlet-side nut fitted along the outer circumferential surface of themeasurement tube to be closer to the inlet than the outlet-side body,the outlet-side nut having on an inner circumferential surface thereofinternal threads fastened to external threads formed on an outercircumferential surface of the outlet-side body; an inlet-side ferruleof resin formed in a cylindrical shape and fitted between the outercircumferential surface of the measurement tube and an innercircumferential surface of an end portion of the inlet-side body facingthe outlet, the inlet-side ferrule being deformed to form a seal area asthe inlet-side nut is fastened to the inlet-side body; and anoutlet-side ferrule of resin formed in a cylindrical shape and fittedbetween the outer circumferential surface of the measurement tubs and aninner circumferential surface of an end portion of the outlet-side bodyfacing the inlet, the outlet-side ferrule being deformed to form a sealarea as the outlet-side nut is fastened to the outlet-side body.
 2. Thethermal flow meter according to claim 1, wherein an end portion of theinlet-side nut facing the outlet is provided with a first recess intowhich an sod portion of the temperature detecting substrate facing theinlet is inserted, the end portion of the temperature detectingsubstrate facing the inlet is fixed to the inlet-side nut by a fillercharged in the first recess, an end portion of the outlet-side nutfacing the inlet is provided with a second recess into which an endportion of the temperature detecting substrate facing the outlet isinserted, and the end portion of the temperature detecting substratefacing the outlet is fixed to the outlet-side nut by a filler charged inthe second recess.
 3. The thermal, flow meter according to claim 1,wherein the measurement tube has on an outer circumferential surfacethereof a flat surface on which the detection surface of the temperaturedetecting substrate is oppositely positioned, and the flat surface andthe detection surface of the temperature detecting .substrate are bondedtogether with an adhesive.
 4. The thermal flow meter according to claim1, wherein a first, distance from the detection surface of thetemperature detecting substrate to an inner circumferential surface ofthe internal flow passage is shorter than a second distance from a topportion of the measurement tube to the inner circumferential surface ofthe internal flow passage.
 5. The thermal flow meter according to claim1, wherein the temperature detecting substrate and the measurement tubeare made of glass.